A head-up display system is used to display information to an observer while permitting the observer to simultaneously view the real world scene. In the past, head-up display systems have been incorporated into aircraft cockpits for displaying important flight information to a pilot. Head-up displays eliminate the need for the pilot to glance downwardly to view the information on an instrument panel.
An important component of a head-up display system is known as the combiner. The combiner is positioned forward of the observer and extends partly across the observer's view of the real world scene. The combiner is constructed to transmit light coming from the real world scene and to reflect light information of a particular wavelength propagating from a luminous source. The light information is projected onto the combiner by way of suitable optical elements. The information-carrying light rays reflected by the combiner are typically collimated to present an image of the information at optical infinity. Accordingly, the observer can simultaneously view the real world scene and the displayed information without changing eye focus. The combiner may be shaped to provide the collimation, or a separate collimating optical element may be employed.
It is known that a particularly effective combiner can be constructed using a hologram or a holographic optical element. Several U S. patents disclose holographic head-up display systems. See, for example, U.S. Pat. No. 4,582,389 entitled "Holographic Device," U.S Pat. No. 4,613,200 entitled "Heads Up Display System with Holographic Dispersion Correcting," or U.S. Pat. No. 3,940,204 entitled "Optical Display Systems Utilizing Holographic Lenses."
Combiners used in prior art holographic head-up display systems most often comprise a hologram recorded in dichromated gelatin and mounted to a planar or nonplanar substrate of either transparent glass or plastic. The combiner is held with suitable support mechanisms within the observer's line of sight. When it is employed in an aircraft cockpit, the combiner is mounted between the observer and the aircraft windshield.
In a motor vehicle, the motorist views the forward outside real world scene through the windshield. Information pertaining to the operational status of the vehicle is typically displayed on an instrument panel located beneath the windshield and out of the motorist's view of the real world scene. It can be appreciated that a head-up display system providing important vehicle information, such as, for example, fuel supply and vehicle speed, positioned within the motorist's field of view through the windshield would permit the motorist to safely maintain eye contact with the real world scene while simultaneously viewing the display information.